The Toxin-Binding Protein of Sugarcane, Its Role in the Plant and in Disease Development

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Abstract
The toxin-binding protein of sugarcane susceptible to eyespot disease also possesses raffinose-binding activity. The K(m)'s for binding are: helminthosporoside (toxin) 6.8 x 10(-5) M, raffinose 2.5 x 10(-5) M, and melibiose 2.6 x 10(-5) M. Evidence obtained by administering [(14)C]raffinose to sugarcane protoplasts suggests that this protein participates in alpha-galactoside transport. Cells from a resistant clone of sugarcane do not possess an active-binding protein, and likewise, do not actively take up raffinose. Interestingly, the K(+), Mg(++) ATPase (ATP phosphohydrolase, EC 3.6.1.3.) on the plasma membrane of the susceptible sugarcane is 30% activated by the toxin at 3 mM. In addition, toxin-treated tissue slices show a rapid uptake of (86)Rb(+)-K(+) which is in agreement with the toxin activation of the membrane K(+), Mg(++) ATPase. Since the ATPase does not directly interact with the toxin, the activation effect occurs by means of the toxin-binding protein. Membrane proteins may be influenced by the toxin-binding protein acting by one of several different mechanisms.